A. Field of the Invention
The invention relates to a method for increasing the permeability of a subterranean formation, wherein the permeability increase is achieved by contacting the formation with an acidic solution to dissolve a portion of the formation. It more particularly relates to an acidizing method of the type employing mud acid.
B. Description of the Prior Art
Numerous procedures for treating wells with siliceous-material-dissolving acids are known. A good discussion of the known art is found in columns 1 and 2 of Templeton et al., U.S. Pat. No. 3,828,854 and in the "Introduction" section of Society of Petroleum Engineers Paper No. 5153, which paper relates to the same invention as the Templeton et al. patent.
Conventionally, siliceous formations have been acidized by contact with mud acid. As used herein, "mud acid" refers to an aqueous solution of hydrofluoric acid and at least one of hydrochloric acid, acetic acid or formic acid; usually, the acid in addition to HF is HCl. As is well understood in the art, the derivation of the HCl and HF is not critical, so that "mud acid" also includes aqueous solutions of chemicals which quickly react to form HCl and HF, i.e., so that by the time the solution reaches the formation, the active ingredients are HF and HCl. The respective concentrations of HCl and HF may vary over wide ranges, with the lower limits being more a matter of practicality rather than operability, and the upper limits being a matter of mutual solubility of the two acids. Thus, any given mud acid solution may have an HCl concentration, by weight, of from about 1 percent or even less up to about 37 percent, and an HF concentration of from about 0.5 percent or even less up to about 25 percent, though as the upper limit is approached for one species, a lesser concentration of the other may be required because of solubility limitations. Most typically, a mud acid is substantially free of other acidic species, consisting substantially of from about 3 to about 25 percent HCl and about 1 to about 10 HF.
In carrying out a mud acidizing treatment, it is fairly common, but not essential, to precede the mud acid with a preflush of aqueous HCl dissolve carbonates in the formation.
In some formations, however, the injection of strong acids such as HCl or mud acid causes a loss in formation permeability and a decrease in well production. Heretofore, such formations could not be beneficially treated with mud acid.
As further background, the use of fluoboric acid in well treating has been previously described. Ayers, Jr., U.S. Pat. No. 2,300,393 teaches treatment with fluoboric acid, optionally with small amounts of HF. Ayers, Jr., warns against using large excesses of HF "as clogging of the treated formation may occur". The clay dissolving efficiency of fluoboric acid alone, however, is relatively poor due to its limited degree of hydrolysis. Ayers, Jr., also teaches the fluoboric acid may be followed by HCl containing no appreciable amount of hydrofluoric acid, or optionally, by a mixture of HCl and fluoboric acids. Bond et al., U.S. Pat. No. 2,425,415 teaches an acidizing procedure wherein the formation is first contacted with a fluoboric acid solution which contains no free HF, but which contains an excess of boric acid, and thereafter with aqueous fluoboric acid containing excess HF. Kingston et al., U.S. Pat. No. 2,663,689 describes the use of boric acid in aqueous HCl-HF to avoid precipitation of insoluble fluoride salts and fluorosilicic acid.